/Engine/Plugins/Online/OnlineServicesNull/Source/Public/Online/AuthNull.h
Auth NULL is implemented in a way similar to console platforms where there is not an explicit login / logout from online services. On those platforms the user account is picked either before the game has started or as a part of selecting an input device.
class FAccountInfoRegistryNULL : public [UE::Online::FAccountInfoRegistry](API\Plugins\OnlineServicesCommon\Online\FAccountInfoRegistry)